Fabrication of the dichroic glass polarizer is known to the art. One of the key processes is heating the elongated metal-halide particles precipitated glass in a reducing atmosphere. The reduction rate varies as the square root of the pressure. Also, the reduction proceeds with a dependence on the square root of time.
One of the important features of a polarizing body is the bandwidth over which the body is effective. This property takes into consideration not only the degree of contrast ratio, but the portion of the spectrum within which the contrast is sufficiently high to be useful. A contrast ratio of 40 dB has been taken as a point of reference for comparison purposes. The lower the reference contrast ratio, the broader the corresponding bandwidth. I have chosen 40 dB contrast ratio because it represents a common high performance value specified for polarizer applications.
The peak contrast ratio wavelength for dichroic glass polarizers is determined by the aspect ratio of the elongated particle. The aspect ratio increases with the degree of stress applied to stretch the glass, and thereby the crystals. The wavelength at which peak contrast ratio occurs increases with the aspect ratio. The precipitated halide particles developed by heat treatment in air atmosphere have a certain size distribution in glass matrix. The aspect ratio of subsequently elongated particles, therefore, has a certain distribution. Thus, the chemically reduced metallic particles have a certain distribution of the aspect ratios. The application wavelength, which is bandwidth, is determined by the combination of the distribution of the peak contrast ratio wavelength by one metallic particle and aspect ratio distribution of metallic particles. Thus, the bandwidth is determined by the summation of the aspect ratios of the metallic particle shapes. The shape of a contrast ratio versus wavelength curve for a polarizing glass is therefore the superposition of the peaks for all the particles. The so-called Center Wavelength (CW) is the application wavelength range in which peak contrast ratio wavelength is optimized with stretching stress and size distribution of silver halide particles. For example, the elongation stress and particles size for a polarizer effective at 1,500 nm are quite different from one effective at 600 nm. In order to broaden the bandwidth, distribution of aspect ratio needs to be broadened. Most applications in the near infra-red (NIR) require an applicable wavelength range of 1,300-1,500 nm. However, other application requires contrast ratio peaks outside this range. For example, peaks as low as 980 nm are used for pump laser application in amplification.
Heretofore, it has been necessary to produce polarizing glass articles on an individual basis. Thus, it was necessary to design a separate set of processing conditions tailored to provide the peak contrast ratio for each application wavelength. Then care had to be taken to control the process quite rigidly.
The maximum bandwidth available heretofore with a commercially practical figure was no more than 200 nm. Broader bandwidth from visible to NIR wavelengths region for dichroic glass polarizers are found in U.S. Pat. No. 4,908,054. In the patent, a contrast ratio greater than 40 dB, is obtained from 610 nm to a 1,060 nm, indicating the bandwidth to be approximately 450 nm. This patent teaches that pressurized hydrogen atmosphere is effective for broadening the waveband. Japanese Patent Office, Kokai Patent Application No. HEI 5 [1993]-208444 describes a contrast ratio greater than 50 dB with the insertion loss less than 0.1 dB is obtained at 1,310 and 1,550 nm and describing wider bandwidth than 200 nm in NIR wavelength region. Glass polarizer with broadband contrast ratio is found in a provisional patent application, Serial No. 60/027,254, filed Sep. 30, 1996, where a heat treatment process for generating silver halide particles is changed in order to impart wider size distribution of the halide particles. This wider distribution of the halide particles results in wider distribution of elongated halide particle, after the stretching step. The wider distribution of the elongated halide particle results in wider distribution of the metallic particles, after the reduction process. Even though this patent does not describe quantitative results on broadened waveband, contrast ratio greater than 40 dB was obtained from about 1,080 nm to about 1,520 nm, indicting bandwidth to be approximately 440 nm. Further, wider bandwidth is found in the provisional patent application, Serial No. (P00210), filed Dec. 4, 1996, where bandwidth, at a contrast ratio greater than 50 dB, is enlarged to 700-900 nm by the reduction under extremely high hydrogen pressure, 100 atmospheres, at a temperature below 400° C. Broadest bandwidth in NIR region in this patent application is 900 nm bandwidth, where the contrast ratio greater than 50 dB is obtained from 600 nm to 1,500 nm. This best result is obtained with two steps reduction process for a CW of about 1,480 nm product, in which the first process is heat treatment in a hydrogen with one atmosphere at 420° C. for 4 hours and the second process is with 100 atmospheres at 350° C. Employment of the extremely high hydrogen pressure would not be a practical process. The purpose of my invention is to broaden the application bandwidth of dichroic glass polarizer with easy practical process.